The modern process of starch wet milling is a series of independent classifications and separations integrated into a single balanced process in which a combination of centrifuges and hydrocyclones are commonly employed to separate product components including germ, fiber, gluten, starch and soluble solids. A great many flow arrangements have been employed over the years, but most users now have basically similar systems utilizing a countercurrent flow of water to the starch fraction. Such a starch wet milling system requires the following treatment stations:
A. Steeping and Germ Separation
B. Fiber Washing and Dewatering
C. Starch-Gluten Separation
D. Starch Washing and Thickening
Each of the product components reports as the product of its respective station with the exception of the soluble solids which "fall out" during steep water evaporation.
The starch consists of spherical granules about 10-15 microns in diameter having a density of about 1.5. The gluten is a flocculant structure of 1-3 microns having a density of about 1.2.
Evaporation of the steep water is carried out to obtain the soluble solids therein in the form of a heavy liquor which is mixed with other products of the process, such as fiber, and dried for use as animal feed.
In the Steeping and Germ Separation Station the corn kernels are first steeped for softening and then cracked in a mill to release the germ. The germ contains the valuable corn oil and is separated from the magma of starch, hulls and fiber in the overflow of a hydrocyclone stage. The germ is washed in washing screens and then exits the system. The essentially germ-free hydrocyclone underflow reports to the Fiber Washing and Dewatering Station.
The Fiber Washing and Dewatering Station includes a grit starch screening stage in which more than half the free starch is removed as undersize and forwarded to the Starch-Gluten Separation (Centrifugation) Station described below. The oversize from the grit starch screening stage is forwarded to a refining mill and then to a plurality of screen stages arranged for countercurrent washing of the fiber with the starch reporting as undersize at a density of 5.degree.Be to 6.degree.Be to the Starch-Gluten Separation Station. The fiber leaves the system at this point reporting to a centrifuge for dewatering and is subsequently dried.
The Starch-Gluten Separation Station accepts the underflow from the grit starch screens and separates the starch from the gluten. The main starch-bearing stream is forwarded to the Starch Washing and Thickening Station while the gluten is forwarded to thickening and drying. Clarified water (free of insoluble solids) is provided by the Starch-Gluten Separation Station for the steeps as is process water (low in solubles) for use in various stages of the system. There is also a recycle stream containing solubles, insolubles and some starch that is sent to the Fiber Washing and Dewatering Station.
The Starch-Gluten Separation Station commonly includes a plurality of centrifuges of the disc nozzle type, the first of which is the Mill Stream Thickener which accepts the feed stream from the grit starch screens underflow and thickens it to a concentration of 9.degree.Be to 12.degree.Be. This thickened stream is forwarded as feed to the Primary centrifuge (a centrifuge accomplishing separation of starch and gluten) into which a wash liquid may be introduced in a wash liquid/draw-off volume ratio of 0.1 to 0.5, the draw-off being the amount of thickened slurry leaving the Primary centrifuge. A wash/liquid draw-off volume ration of 0.5 approximates the maximum ratio formerly attainable with available centrifuges in the corn wet milling process. The volume of wash liquid addition improves the protein (gluten) recovery from 35.+-.10% to a maximum of 50.+-.10%.
The low protein recovery of the primary separator, especially when using larger size disc-nozzle centrifuges, causes protein to build up in the system and increases the runaround inside the loop including the fiber washing stages and the starch washing stages. A tight control on underflow density of the washing stages is a necessity in order to meet the finished starch quality requirement (less than 0.3% protein).
In the corn wet milling process, there are many advantages from the process, economic and operational points of view to be derived from an improvement in protein recovery to as close to 100% as possible in the primary stage, and by avoiding protein accumulating and recycling in the process flows.
The Starch Washing and Thickening Station is a multiple-stage countercurrent washing system using hydrocyclones which remove solubles together with the remaining insoluble protein and fine fiber in the feed stream as the final starch product is concentrated. The overflow stream from the Starch Washing and Thickening Station has the lowest soluble solids concentration of any stream in the process (except fresh wash water) and is returned to the Starch-Gluten Separation Station as process water. A system of the type described above is outlined in detail in an article entitled "Integrated Starch Wet Milling Process" by T. H. Bier, J. C. Elsken and R. W. Honeychurch, published in Die Starke 26 Jahrg. 1974/No. 1, Pgs. 23-28.
In co-pending U.S. patent application Ser. No. 612,044 for "High-Rate Washing Centrifuge", filed Nov. 13, 1990, there is disclosed an improved disc-nozzle centrifuge with structure permitting the introduction of very much larger amounts of wash liquid into the centrifuge than had been possible heretofore. Wash liquid in volume of from over 0.5 to 3 times the underflow draw-off volume is introduced directly into the rotor/separation chamber of the centrifuge simultaneously with the return flow of recycled underflow and results in displacement washing of the feed. The liquid originally associated with the feed is largely displaced to the overflow.
In addition to the washing function, substantial improvement in classification of solids is effected due to the high rate of return flow and wash liquid into the separation chamber. The high upflow wash elutriates the bed of solids within the rotor/separation chamber and lifts the fines out of the fluid bed and sweeps them to the overflow.
The disclosure of application Ser. No. 612,044 is incorporated herein by reference.
It is an object of the invention to provide a starch wet milling process which incorporates an improved Starch-Gluten Separation Station and process.
It is a further object of the invention to utilize the high-rate washing centrifuge in the Starch-Gluten Separation Station of the starch wet milling process to obtain improved washing of the starch and a sharp classification into starch and gluten.